As there is a "buddy" list, I would also like to have an "enemy" list in another tab. Members of the list could be added/removed with /addenemy and /removeenemy, or /aenemy and /renemy.
I think a list like this could serve a few different purposes in the game: It would help players remember who to be wary of and know when they are online. Also it could serve players as a "hit list" of other players that they want to attack/PK.
Attached is a mockup.